1. Field of the Invention
The present invention relates to a sliding or rolling contact element which is surface treated in order to reduce friction and wear between the contacting surfaces of two elements in sliding or rolling contact with each other and to increase the load carrying capacity and seizure resistance of each of the two elements. It also relates to a rolling bearing employing the rolling contact element.
2. Description of the Related Art
In normal bearing lubrication, the friction coefficient between two sliding or rolling contact surfaces of metal bodies changes with the absolute viscosity of lubricating oil used therewith, the bearing load and the rotational speed between the two sliding or rolling contact surfaces. In complete or full fluid lubrication in which a thick and continuous film of oil is formed in the interface between the two sliding or rolling contact surfaces, contact between the underlying surfaces of the two metal bodies will almost never take place so that the friction coefficient of the two sliding or rolling contact surfaces is very low. In mixed lubrication in which metal-to-metal contact partially takes place or in boundary lubrication in which a lubricant oil film is broken to thereby cause a few patches where metal-to-metal contact takes place, it is possible that surface damage, e.g., seizure of the sliding or rolling contact surfaces to be lubricated will take place. In order to prevent this damage, various types of lubricant additives (e.g., an oiliness agent, an extreme-pressure agent and an antiwear agent) have been employed.
The interface between the contacting surfaces of the two metal bodies is at high temperature and high pressure so as to be in a high reactive condition. A lubricant additive (an organic compound with chlorine, sulfur or phosphorus, e.g., chlorinated paraffin, dibenzyl sulfide or tricresyl phosphate) introduced into the area of contact readily reacts with a bearing metal.
As a result of this reaction, inorganic or organometallic compounds with each element (e.g., metal chloride, metal sulfide, metal phosphate, etc.) are produced on the contacting surfaces of the two metal bodies. The reaction products are considered to prevent the seizure and wear between the contacting surfaces of the two metal bodies. However, an immediate effect of the lubricant additive cannot be achieved since a considerable amount of time is required for the start of a reaction between the lubricant additive and the contacting or frictional surfaces of the two metal bodies.
In the prior art, there are some examples in which a sliding or rolling contact element is previously surface treated with a lubricant additive, e.g., tricresyl phosphate (i.e., TCP).
However, there has not yet been an established theory of the effect when the sliding or rolling contact element is previously surface treated with a lubricant additive, so that this effect can only be variously estimated.
In particular, this effect under a severe condition in which a supply of lubricating oil is frequently interrupted during operation of the sliding or rolling contact element has not been discussed. That is, the thickness of a surface treated layer of the lubricant additive formed on the sliding or rolling surface by the prior art is as thin as less than 0.05 .mu.m measured by X-ray photoelectron spectroscopy (i.e., XPS, described later). The present invention carried out a superhigh speed four ball test in order to study an improvement in the lubricating property of a bearing caused by such a thin surface treated layer. This superhigh speed four ball test is a kind of extreme-pressure test essentially equal to ASTM D-2783. The rotational speed of the vertical shaft of a superhigh speed four ball tester can be increased up to 20,000 rpm while that of a four ball tester according to ASTM D-2783 can be increased up to 1,770 rpm. The superhigh speed four ball test disclosed that the surface treated layer as thin as less than 0.05 .mu.m would not improve the extreme-pressure property, in particular, under the severe condition in which, for example, the supply of lubricating oil was interrupted.
Another surface treatment of the bearing metal with an inorganic compound, e.g., zinc phosphate or manganese phosphate instead of the above mentioned reaction product of the lubricant additive, has been attempted. In this case, there is a problem in that this inorganic compound causes corrosion in the bearing metal surfaces. In addition, the thickness of the surface treated layer of this inorganic product is above 0.5 .mu.m. The superhigh speed four ball test applied to this surface treated layer of the inorganic product disclosed that the surface treated layer of this inorganic product failed to improve the extreme-pressure property.
Thus, the prior art failed to sufficiently disclose an effective thickness of the surface treated layer formed on each of the two contacting surfaces, so that a sufficient improvement in the lubricating property caused by the surface treatment applied to the sliding or rolling contact element was not achieved.